In recent years, with a trend toward higher performance and toward downsizing of an image pickup apparatus employing a solid state image pickup element such as an image sensor of a CCD (Charged Coupled Device) type or an image sensor of a CMOS (Complementary Metal oxide Semiconductor) type, a cell-phone and a mobile information terminal each being equipped with an image pickup apparatus are spreading. Demands for further downsizing and further higher performance for an image pickup lens equipped on the image pickup apparatus are increased. As the image pickup lens to be used in this way, an image pickup lens in a 4-element structure is suggested, because it can exhibit higher performance when comparing with a lens in a 2-element structure or a 3-element structure.
As the image pickup lens in the 4-element type, there is disclosed an image pickup lens of the so-called Inverted Ernostar type that is composed of a first lens having positive refractive power, a second lens having negative refractive power, a third lens having positive refractive power and a fourth lens having positive refractive power, in this order from the object side, aiming to exhibit high performance (For example, JP-A No. 2004-341013).
Further, there is disclosed an image pickup lens of so-called telephoto type that is composed of a first lens having positive refractive power, a second lens having negative refractive power, a third lens having positive refractive power and a fourth lens having negative refractive power, in this order from the object side, aiming to shorten the total length which is a distance from the lens surface on the object side of the total image pickup lens to the focal point on the image side on the optical axis (for example, JP-A Nos. 2002-365529, 2002-365530, and 2002-365531).
Since the image pickup lens described in the aforesaid JP-A No. 2004-341013 is of the Inverted Ernostar type, the fourth lens is a positive lens. In such image pickup lens, a position of a principal point of the optical system comes to the image side and its back focus becomes to be longer compared with an occasion where the fourth lens is a negative lens as in the telephoto type, which is disadvantageous for downsizing. In addition, only one lens among four lenses in the image pickup lens has negative refractive power, which makes correction of Petzval's sum to be difficult and causes a fear that excellent performance cannot be secured on a peripheral part of an image which is picked-up by the image pickup lens.
In the image pickup lenses described in JP-A Nos. 2002-365529, 2002-365530, and 2002-365531, an angle of view for photographing is small and aberration correction is insufficient. Further, when the total lens length is shortened, there is caused a problem that the image pickup lens hardly cope with an image pickup element with larger number of pixels due to deterioration of lens performance caused by the shortened total lens length.